1. Field of the Invention
The present invention relates to a data processing system and, more particularly, to a data file in a word processor or a personal computer having functions to input, register and read document data or the like.
2. Description of the Prior Art
In a personal word processor or an MSX (i.e., the unified standards for personal computers by Micro Soft and ASCII) personal computer which has seen substantial success as a small-sized documentation system in recent years, a tape recorder (which may be called a data recorder or a cassette tape recorder) is used as one of external memories of a file system.
The FSK (i.e., Frequency Shift Keying) modulation process is generally adopted as a data modulation process when data is to be stored in the tape recorder, because it is highly resistant to noise. The FSK modulation process is a frequency modulation process, by which frequencies are shifted in a manner to correspond to the data "0" and "1", respectively. In a demodulation process, reproduced analog signals from the tape recorder are subjected to A/D (i.e., analog-to-digital) conversion, and the reproduced FSK modulation digital signals are sampled.
Incidentally, the use of an audio cassette tape recorder for filing data is exemplified in Japanese Patent Laid-Open No. 929/1980.
For the demodulation, a sampling is made in the MSX personal computer, for example, noting the pulse width of the A/D converted reproduced FSK demodulation digital signals. Specifically, whether or not the data is at "0" or "1" is judged by grasping the changing points of said reproduced FSK modulation digital binary signals from the high to low levels and the low to high levels to continuously count the time periods of the high and low levels. This sampling method is restricted by a hardware circuitry that said reproduced FSK modulation digital signal corresponding to 1-bit data must have a duty of 50% when the 1-bit data is to be sampled.
However, it is difficult to make said reproduced FSK modulation digital signal of the 1-bit data into a signal of the 50% duty partly because the duty is not the value of 50% if the FSK modulation digital signal of the 1-bit data is D/A (i.e., digital-to-analog) converted when the prepared data is to be stored in the tape recorder, although the FSK modulation digital signal of the 1-bit data can have a substantially constant period, and partly because the reproduced output analog signal from the tape recorder is processed to have hysteresis characteristics so as to prevent the chattering when it is A/D converted into the reproduced FSK modulation digital signal. This raises a problem that the ratio of sampling mistakes increases.
If the data transfer rate is to be improved, there arises another problem of the data sampling time.
Therefore, the inventors of the present invention have tried to solve those problems by grasping either of the changing points of said reproduced FSK modulation digital binary signals from the high to low levels and from the low to high levels to count the changing points and their time periods.
However, the existing tape recorder takes consideration of the recording level and the frequency characteristics but not the phase. Specifically, the tape recorder is divided into two types, in which the phases upon the recording and reproduction are in and opposite. Therefore, the method of grasping only either the changing point of the reproduced FSK modulation digital binary signals from the high to low levels and the low to high points to count the changing points and their time periods is troubled by the phase, and the use of such tape recorder may disable the signals to be correctly sampled. As a result, that sampling is troubled by the fact that it cannot be used except in the tape recorder having a phase inverting switch.